1. Field of the Invention polyisocyanates to polyisocyanurates and
This invention relates to the preparation of polymer foams and is more particularly concerned with novel catalyst combinations for the trimerization of polyisocyanurates ad their utilization in the preparation of polyisocyanurate foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having high resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U.S. Pat. Nos. 3,516,950, 3,580,868, 3,620,986, 3,625,872, and 3,725,319. The process described in U.S. Pat. No. 3,745,133 discloses the use of a combination of an epoxide and a tertiary amine as cocatalysts.
The above methods require the foaming process to proceed in conjunction with at least two polymer forming reactions; namely, the isocyanurate formation arising from the homopolymerization of the isocyanate employed, and the minor amount of polyurethane formation arising from the reaction of a polyol with the isocyanate. Difficulties arise, particularly in commercial applications, due to the variation in relative rates of the two polymerization reactions. The polyurethane formation frequently initiates before the trimerization formation thus giving two separate rise steps in the overall foam rise profile.
These difficulties are accentuated in the case of application of polyisocyanurate foams by spray techniques. The latter normally require that the polyisocyanurate foam forming mixture be supplied in only two streams to the mixing and spraying head. Further these two streams are normally so formulated, by appropriate preblending of components, that they can be brought together either in equal ratio by volume or with the polyisocyanate containing component in twice the ratio by volume of the other component. These requirements are largely dictated by the design of spray equipment currently available.
It would obviously be highly desirable to use the same two components for either of the above ratios or any ratios intermediate therebetween. However, because of the critical role which the nature and proportion of catalyst plays in the preparation of polyisocyanurate foams, it has not hitherto been possible to devise a catalyst which would perform satisfactorily at the different catalyst levels which result when the component ratios are changed in the above manner. Accordingly it has hitherto been necessary to provide substantially different formulations for operation at the different component ratios. Further it has even been very difficult to formulate a system which could be operated satisfactorily when the two components of the system are employed in equal ratio by volume.
We have now found a catalyst system which is free from the above difficulties and which is particularly useful in spray foam systems.